1. Field of the Invention
The present invention relates generally to methods and devices for the non-destructive evaluation of materials. The present invention relates more specifically to a magnetostrictive sensor based system for the long-range guided-wave inspection of longitudinal structures and a method for automatically differentiating reflected signals from intended geometric elements within the structures (such as welds) from signals generated by unintended geometric features in the structures (such as defects).
2. Description of the Related Art
Introduction
The ongoing ability of structural components to function in their intended manner often depends upon the maintenance of their material integrity. Various techniques are used to investigate and monitor the integrity of longitudinal structural objects. Non-Destructive Evaluation (NDE) techniques are important tools to accomplish this investigation and monitoring. NDE techniques range from ultrasonic testing to electromagnetic (EM)•testing systems and methods. A highly beneficial feature for some NDE techniques is their ability to investigate and monitor a large (especially long) structure from a single or small number of points on the structure. Many such “large structures” are longitudinal in nature (pipes, cables, tubes, plates, and conduits for example). Such structures present specific problems for the investigation and/or monitoring of locations as much as 100 feet from the placement of an NDE type sensor system.
Long-Range Guided-Wave Inspection
Long-range guided-wave inspection of structures is a recently developed NDE inspection method that can examine a long length (such as 100 feet) of structure (such as pipes, tubes, steel cables, and plates) quickly, and therefore economically, from a given sensor location. At present, there are two well established guided-wave inspection technologies. One is referred to commonly as magnetostrictive sensor (MsS) technology, and has been pioneered by Southwest Research Institute (SwRI) of San Antonio, Tex. (SwRI is the Assignee of a series of U.S. Patents covering MsS based Long-Range Guided-Wave Inspection techniques, including U.S. Pat. Nos. 5,456,113, 5,457,994, 5,581,037, 5,767,766, 6,212,944, 6,294,912, 6,396,262, 6,429,650, 6,624,628, 6,917,196, and as well as additional pending patents).
An example of the functionality of MsS Systems, as described above, can be found in U.S. Pat. No. 6,917,196 issued to Kwun et al. on Jul. 12, 2005 entitled Method and Apparatus Generating and Detecting Torsional Wave Inspection of Pipes and Tubes. This patent describes one approach for implementing MsS Techniques for the NDE of pipes or tubes. In this case, a MsS generates guided waves which travel in a direction parallel to the longitudinal axis of the pipe or tube. This is achieved (in this particular sensor system) by using a magnetized ferromagnetic strip pressed circumferentially against the pipe or tube. The guided waves are generated in the strip, are coupled to the pipe or tube, and propagate along its length. Detected guided waves are coupled back to the thin ferromagnetic strip and may include reflected waves representing defects in the pipe or tube.
A second NDE technique used in conjunction with longitudinal structures is commonly referred to as Lamb wave inspection technology. Commercial systems implementing such techniques are marketed under the names Teletest® and Wavemaker®. These systems have been developed by the Imperial College of Science, Technology of Medicine of London, England. These techniques are typified by the system described in U.S. Pat. No. 6,148,672 entitled Inspection of Pipes issued to Crawley et al. on Nov. 21 2000 and assigned to Imperial College of Science, Technology of Medicine. The patent describes an apparatus and a method for inspecting elongate members, especially pipes, using Lamb waves. The apparatus and method provide an axi-symmetric Lamb wave of a single mode in one direction along the pipe. A receiver is provided to receive the Lamb wave after its passage along the pipe and converts the received wave for storage processing and analysis to determine whether or not there are faults present in the pipe. The apparatus includes at least one, and usually several, excitation rings, each having a number of Lamb wave exciters deployed in equiangular spacing and a ring clamping structure whereby each exciter can be pressed with equal force against the surface of the pipe under inspection.
The MsS based systems described above generate and detect guided waves in ferromagnetic materials (such as carbon or alloyed steel) without requiring direct physical contact to the material. The Lamb wave based systems on the other hand, generate and detect guided waves by coupling the waves to an array of piezoelectric sensors in direct physical contact to the material. The MsS is applicable for inspection of various structures including pipes, tubes, plates, and steel cables, whereas the Lamb wave method is primarily for inspection of pipe from the outside. Both technologies are now in commercial use.
The advantages of using the magnetostrictive effect in NDE applications include; (a) the sensitivity of the magnetostrictive sensors, (b) the mobility of the magnetostrictive sensors, (c) the absence of a need to couple the sensor to the material being investigated, (d) the long-range of the mechanical waves in the material under investigation, (e) the ease of implementation, and (f) the low cost of implementation. The use of magnetostrictive sensors (MsS) in the NDE of materials has proven to be very effective in characterizing defects, inclusions, and corrosion within various types of ferromagnetic and non-ferromagnetic structures. Since guided-waves can propagate long distances (typically 100 feet or more) the magnetostrictive sensor technology can inspect a global volume of a structure very quickly. In comparison, other conventional NDE techniques, such as ultrasonics and EMAT current, inspect only the local area around the sensor.
Signal Analysis—Separation of Defect Signals from Geometric Feature Signals
With guided-wave inspection techniques, a pulse of guided-wave of a given frequency and wave mode is launched along the length of a structure, and signals reflected from anomalies in the structure are detected. Anomalies in the structure that cause a wave reflection include defects (such as corrosion wall loss areas and cracks) and geometric features in the structure (such as welds, weld attachments, and clamps). When a long section of structure, such as in a piping network in a refinery, chemical plant, or power-generation plant, is examined using the long-range guided-wave inspection technology, the test data contain signals produced by geometric features as well as those produced by defects (if defects are present in the structure). In order to find defects and to reduce false calls, the signals produced by geometric features must be properly identified and distinguished within the test data.
If the structure under investigation can be visually examined for the locations of geometric features, proper identification of the geometric feature signals in the test data can readily be made. In cases where the section of the structure examined is hidden from view (for instance, the structure is covered with insulation, buried, or is inside another structure), proper identification of geometric feature signals can be very difficult. In these cases, not only could analysis of the test data be time consuming, but data analysis results could be questionable unless confirmed by exposing the hidden section to direct examination.
In order to improve the reliability of the long-range guided-wave inspection results and, at the same time, reduce the time and expense of the data analysis and confirmation, a method that can automatically separate and identify geometric feature signals from defect signals is needed. The invention disclosed below describes a method and algorithms for achieving automated identification and differentiation of geometric feature signals from defect signals.